There's a bug in 'something' that causes the gnome mixer applet to lose its mind.

It's sort of hard to explain.  I'm listening to something, and think "ooh, this is a bit loud, I'll turn it down" or "this rocks! more sounds!" and head to the applet to make with the slideys.  I mousewheel over the slider in the appropriate direction, and at first it starts to do what you'd expect. Then, everything just goes completely bonkers.  The left and right sliders which were previously sliding in unison have become separated.  Worst still, one of the sliders has jumped to 0.  Just when I think things can't get much worse than this, the other slider does the same, and the applet helpfully thinks "oh, you've slid both sliders to zero. you must want to mute the audio!" and a honking big X appears over the speaker icon.

So then I have to click the unmute button, click the 'relink' button to make the sliders slide together again, and then retry that sliding thing.

My first guess was that the mouse wheel was sending a 3rd mousebutton press event which was confusing things, but that isn't the case. I can also reproduce this with just grabbing one slider with the left mouse, and wiggling it up and down until the two become detached.